A known system for surveying the structure beneath the seabed using a sub-bottom profiler 15 is shown in FIGS. 1(a), (b). In this system, a transmitter 20 and a receiver 30 are hull mounted to a survey vessel with the transmitter 20 being upstream with respect to the direction of travel of the survey vessel. The transmitter 20 transmits pulses that travel towards the seabed. Since sound energy is reflected off the boundaries between layers of different densities (and hence acoustic impedance), as a transmitted pulse encounters the boundary between the water and the surface of the seabed, some of its energy is reflected and some of its energy penetrates further into the seabed. As its energy penetrates further into the seabed, further reflections occur as boundaries between layers of, for example, clay, sand and other sediments are encountered. The receiver 30 receives the reflections that allow an image of the structure beneath the seabed to be built up. FIGS. 2 (a), (b) illustrate the continued operation of the known system. It may be seen that the distance on the seabed between the reflection point of successive pulses, dx, i.e. the density of the data collected, is governed by the pulse repetition rate and the speed of the survey vessel. For a pulse repetition rate of 450 ms and a survey vessel moving at 4 knots (2 m/s), the resulting dx is 0.9 m.
The present invention stems from the inventors' realization that surveying capability is fundamentally constrained by the key physical parameters of the survey, namely the deepest water depth and the penetration beneath the seabed, which place a limit on the maximum pulse repetition rate.